Polylactic acid (PLA) has such a molecular structure as shown in FIG. 14. PLA is dehydrated condensation polymer obtained through ring-opening polymerization of lactide, which is a cyclic dimer of lactic acid. Since lactic acid contains asymmetric carbon, it has chirality. Therefore, L-form and D-form are present in PLA, whose polymers are referred to as poly L-lactic acid (PLLA) and poly D-lactic acid (PDLA), respectively. Main chain of PLLA has left-handed helix structure, and main chain of PDLA has right-handed helix structure. Polylactic acid derived from lactic acid synthesized by microorganism is mostly in L-form and, currently mass-produced and used PLA is PLLA.
As described, for example, in Patent Literature 1 and Non-Patent Literature 1, a stretched PLA film exhibits piezoelectricity. According to Non-Patent Literature 1, point group of PLLA crystal is D2, and has such component as shown in FIG. 15(a) as piezoelectric tensor.
PLLA is helical polymer having dipoles of large value in the helix axis direction (C-axis direction). Crystal structure of PLLA is packed with dipoles in the C-axis direction facing opposite directions alternately. Therefore, macroscopically, dipoles in the C-axis direction are cancelled with each other to be zero. Therefore, assuming that a PLLA sheet is stretched in “3” axis direction as shown in FIG. 15(b), the piezoelectric tensor of the stretched, uniaxially-oriented PLLA sheet comes to have such component as shown in FIG. 15(c) as a result.
As described, for example, in Patent Literatures 1 and 2, piezoelectric phenomenon of conventional piezoelectric PLLA mainly comes from d14 shown in FIG. 15(c), of which value is about 10 to 20 pC/N. The piezoelectric constant of PLLA is distinctively high among polymers.
On the other hand, PZT as a representative example of ceramic piezoelectric body commercially available at present has as high a value as d33=300 to 700 pC/N, and it is applied to various actuators, piezoelectric buzzers and piezoelectric speakers. PZT, however, is lead-containing material and, from the viewpoint of environmental protection, lead-free piezoelectric material has been desired in the market. Further, among inorganic piezoelectric materials, ceramics are dominant, of which manufacturing cost is high and which inevitably involves disposal by landfill. From the foregoing, polymer piezoelectric material that can be manufactured at a low cost and is easily disposable has been desired. No material having piezoelectric constant comparable to PZT has said to be found.
PVDF (polyvinylidenefluoride) and PLLA are considered promising as polymers having very high piezoelectric constants. Particularly, as can be seen from FIG. 14, PLLA contains only C, O and H as constituent elements and, therefore, it will not emit any harmful substance when incinerated. Further, PLLA is biodegradable plastic that can be fully decomposed to water and CO2 through two steps of decomposition process of hydrolysis and microbial degradation. At present, raw material is corn starch, and petroleum oil is not at all used as the raw material. Except for CO2 derived from energy used in the manufacturing process, PLLA itself does not increase CO2 in the air after decomposition, since original material of PLLA is CO2 in the air. This is the reason why PLLA is considered carbon-neutral, and it attracts attention as environmentally friendly material.
It is noted, however, that piezoelectric constant of PLLA is, for d14, at most 20 pC/N, which is very low as compared with the piezoelectric constant of PZT. In order to use PLLA as a replacement of PZT, it is necessary to apply very high voltage at the time of operation. Therefore, conventionally, replacement to PLLA has been very difficult.
PTL 1: Japanese Patent Laying-Open No. 5-152638
PTL 2: Japanese Patent Laying-Open No. 2005-213376
NPL 1: Yoshiro TAJITSU, “Poly Nyusan Maku no Hikari/Denki Kinou (Optical/Electric Functions of Polylactic Film)”, Mirai Zairyo, July 2003, Vol. 3, No. 7, pp. 16-25.